1. Field Of The Invention
The invention is concerned with optical devices which use liquid crystal materials for recording, storage, and display of graphical patterns.
2. Description of the Prior Art
Recent advances in laser technology and liquid crystal recording materials have made possible the development of high resolution graphical display devices which have distinct advantages over devices utilizing a cathode ray tube. Among such advantages are high resolution and image brightness, absence of flicker, and the capability of large screen display; devices enjoying these advantages are surveyed, for example, in "Infrared Laser Addressing of Media for Recording and Displaying of High-Resolution Graphic Information," Proceedings of the IEEE 61 (1973), pp. 1007-1013. To store graphical information such devices use a thin layer of a liquid crystal material contained between a pair of flat electrodes which are transparent to visible light but which absorb infrared radiation. A suitable electrode material is In.sub.2-x Sn.sub.x 0.sub.3-y which can be conveniently deposited by sputtering on fused silica substrates. In the following, an assembly consisting of a liquid crystal material contained between a pair of visibly transparent, infrared absorbing electrodes is called a liquid crystal cell.
Projection of patterns recorded in a liquid crystal cell can be effected in a manner analogous to the projection of a photographic slide in a projector utilizing a projector lamp and condenser and projection lenses. However, recording of patterns in a liquid crystal cell is effected thermally rather than optically and requires no chemical processing. Typically, while the bulk of the liquid crystal material is held at a temperature at which it is in a transparent, ordered liquid crystal phase, an isotropic phase is created by localized heating of the liquid crystal material. If the heated areas are allowed to cool at an appropriate rate, these areas do not order upon cooling to the temperature of the bulk liquid crystal phase and act as light scattering centers. Conversely, if the bulk of the material is in a light scattering state, local heating followed by cooling in the presence of an electrical field applied across the liquid crystal layer can be used to record patterns by locally eliminating light scattering centers.
Thermal writing is conveniently effected by a focused x-y deflected, intensity modulated infrared laser beam which locally heats the infrared absorbing electrodes. By moving the laser beam across the liquid crystal cell, any desired graphical pattern can be recorded.
Two types of liquid crystals have been used successfully in such display devices. Both types, "cholesteric" as well as "smectic" liquid crystals, have elongated, cigar-shaped molecules; they differ, however, in the arrangement of the molecules in thin liquid crystalline films. Specifically, for a uniformly ordered thin film of a Smectic liquid crystal material, molecules in the transparent state are arranged into parallel layers with the major axis of each molecule oriented, on the average, perpendicular to the layer. On the other hand, molecules in the transparent, uniformly ordered state of a cholesteric liquid crystal material have a helical ordering, the molecules on the average lying in planes normal to the helical ordering axis. In either case, uniform alignment in the liquid crystal phase is induced by a coating of a coupling agent applied to the substrate surfaces between which the liquid crystal material is contained. In F. J. Kahn, "Orientation of Liquid Crystals by Surface Coupling Agents," Appl. Phys. Lett. 22 (1972), pp. 386-388, a silane coupling agent is recommended for this purpose.
A specific example of a cholesteric liquid crystal is the material used for experiments described in H. Melchior et al., "Thermally Addressed Electrically Erased High-Resolution Liquid Crystal Light Valves," Appl. Phys. Lett. 21 (1972), pp. 392-394 and consisting of 90 percent N-(p-methoxybenzylidene)-p-n-butylaniline (MBBA) and 10 percent cholesteryl nonanoate (CN). An example of a smectic liquid crystal is N-(p-cyanobenzylidene)-p-n-octylaniline (CBOA) used in F. J. Kahn, "Laser-Addressed Thermo-Optic Smectic Liquid-Crystal Storage Displays," Appl. Phys. Lett 22, (1973) pp. 111-113. Liquid crystal cells using a smectic rather than a cholesteric liquid crystal material are more versatile due to a local erase capability. While erasure of scattering centers in a cholesteric liquid crystal merely requires the application of a suitable voltage to the electrodes containing the liquid crystal material, erasure of a pattern recorded in a smectic liquid crystal requires reheating the recorded pattern into the isotropic phase followed by cooling to the initial temperature of the smectic liquid crystal phase in the presence of a suitable voltage applied across the liquid crystal layer.